Patent GB-1495787-A (GB)
Title: Signal synchronization acquisition in a satellite communication system
IPC: H04B7/204
Filed: 1975-02-26
Abstract: 1495787 TDMA communication systems FUJITSU Ltd 26 Feb 1975 [26 Feb 1974 (2)] 8108/75 Heading H4M A synchronization acquisition control circuit for a remote station in a satellite communication system, wherein the satellite has a TDM operational cycle, preset at the satellite, which includes a synchronization time window such that the satellite can retransmit to the station that proportion of a synchronization acquisition signal burst which is received at the satellite within the time window, incorporates an acquisition signal generator which provides a predetermined kind of signal burst made up of two differing signals transmitted successively. Retransmitted signals are detected at the ground station and in dependence thereon the timing of succeeding acquisition signal bursts is shifted until the retransmitted signals are a predetermined part of the burst. The timing shifts are in accordance with one of two modes of operation. In the first or search mode, relatively large timing shifts are made until more than a predetermined minimum proportion of a burst is detected. Thereupon a switchover occurs to a second or synchronizing mode in which relatively small shifts are made until a synchronized condition is reached. If a nearby ground station is communicating via the satellite it is desirable that the acquisition signal bursts should be at very low level to avoid interference. In this case the returned burst may be heavily masked by noise. To enable the burst pulses to be recognized in this case, the acquisition signal is repeatedly transmitted at each time shift and samples of the returned signal at different time instants are summed. The sums in excess of a threshold are regarded as defining the returned pulse. The control circuitry at a ground station is shown generally in Fig. 6. The acquisition signal burst is formed by successive bursts at two different frequencies F1, F2 generated at 16, 17 and switched in succession at 18, 19 to a hybrid circuit 20. If a low level acquisition burst is to be transmitted switches 21, 30 are set to the positions shown to route the signal at the transmission end via an attenuator 23 before transmission at 25 to the satellite. Such part of the burst as is returned and received at 26 passes (in this operational mode) via an amplifier 29 to a pair of band pass filters 32, 33 timed to the two frequencies F1, F2 and thence to detectors 34, 35. The outputs from these pass to comparators 36, 37 and thence to pulse width detectors 38, 39, which in this mode of operation accumulate repeated samples of returned acquisition bursts all transmitted with the same timing. Having established the widths of the two returned frequency bursts at 48, 49, these are applied to a phase error detection and control circuit 40 where they are compared with the predetermined burst to be returned when the ground station is correctly synchronized. The output from this controls a transmit phase control circuit 41 which adjusts the timing of the transmitted acquisition burst by control of the switches 18, 19. This will be in jumps of 300 ms. or 6 Ás, depending on whether the system is in a search mode or sync mode as determined by whether or not any part of the burst is retransmitted and detected. A sync window may be applied to the system as an input to controller 44 by reference to another station. Details are provided of the signal detection logic arrangements which respond to the repeated reception of the noise masked low level bursts and the control logic for adjusting the phase of transmitted bursts (Figs. 7-9, not shown).